1. Field of the Invention
The present invention relates to a wire electrical discharge machine with an automatic wire feeder that performs electric discharge machining on a work.
2. Description of the Prior Art
Conventional wire electrical discharge machines are known to have a construction shown in FIG. 6. This type of wire electrical discharge machine uses a feed pipe 5 vertically movable along a head 10 to guide and insert a wire electrode 1 supplied by the wire electrode feeder into a start hole and machining path holes 26 in a work 8 secured by a clamp 22 on a support base 21. The head 10 is mounted with guide rods 24 each formed with a feed screw 35. A holder 20 has the feed pipe 5 secured thereto so that the feed pipe 5 extends vertically downward. The holder 20 is driven vertically along the guide rods 24 of the head 10 by rotating the feed screws by a motor (not shown). The wire electrode 1 that was used for electrical dischange machining of the work 8 is pulled out by pullout rollers 12 installed downstream of a lower wire head 7. The feeding of the wire electrode 1 is detected by a wire break sensor 62 and is performed by a pair of anneal rollers 13 provided to the holder 20 that vertically reciprocates along the head 10 or by a pair of common rollers 14 provided downstream of a cutter 16, which is installed in a lower part of the head 10 to cut the wire electrode 1. The cutter 16 cuts the wire electrode 1 to trim the front end of the wire electrode 1 so that it can be passed through an upper wire head 6, a hole 26 of the work 8 and a lower wire head 7. The wire electrical discharge machine of this kind is disclosed in, for example, Japan Patent Laid-Open No. 145215/1990.
The above-mentioned wire electrode feeder is disclosed in Japan Patent Publication No. 15125/1994. The wire electrode feeder includes a pair of guide rollers arranged below the lower wire head to clamp and guide the wire electrode, pullout rollers located by the side of the guide rollers to draw out the wire electrode, and a belt wound on the guide rollers and the pullout rollers. The wire electrode, worn after being used for electrical discharge-machining of the work, is drawn out through the lower wire head, wound around the guide rollers, clamped and moved by the belt, and then pulled out by the pullout rollers for recovery.
Another type of wire electrode feeder is disclosed in Japan Patent Publication No. 75808/1994. The wire electrode feeder incorporates a direction change roller disposed below the lower wire head. This feeder also includes a guide pipe to feed out the wire electrode, after having its direction changed by the direction change roller, by water flow, and pullout rollers to clamp and pull out the wire electrode discharged from the guide pipe.
With the conventional wire electrical discharge machine, however, when the wire electrode situated between the anneal rollers and the common rollers is clamped by the anneal rollers and subjected to annealing, the wire electrode is formed with an impression at the clamped portion. When the wire electrode with an impression is automatically supplied and passed into a hole in the work, it needs to be moved down through the hole at a very slow speed, making it impossible to shorten the wire electrode feeding time. That is, the annealing of the wire electrode involves holding the wire electrode by the anneal rollers and driving the common rollers located below the anneal rollers in forward direction to tense the wire electrode. The wire electrode is therefore formed with a large impression where the anneal rollers clamp it. Because the impression is located downstream of the annealed portion of the wire electrode, it becomes a hindrance to the passage through the upper wire head, lower wire head and the hole in the work, preventing smooth feeding of the wire electrode.
As the wire electrical discharge machine becomes large, the distance from the wire electrode direction change roller in the wire electrode recovery device located below the lower wire head to the pullout rollers installed outside fluid tank of the wire electrical discharge machine increases. If the distance from the direction change roller to the pullout rollers is long and if the speed at which the wire electrode is fed out is equal to the speed at which the wire electrode is passed through the hole in the work, the time it takes for the wire electrode to reach the pullout rollers becomes large.
In the conventional wire electrode recovery device of a wire electrical discharge machine shown, for instance, in FIG. 6, because in feeding the wire electrode 1 the anneal rollers 13 or common rollers 14 are driven and because in annealing the wire electrode 1 in a tensed state the wire electrode 1 is held immovable by the anneal rollers 13 while the common rollers 14 are driven, it is necessary to control the driving of the anneal rollers 13 and the common rollers 14 independently. For example, as shown in FIG. 5, to drive the common rollers 14 requires rotatably supporting the common rollers 14 on arms 43 through bearings 51, mounting the common rollers 14 to drive shafts 52, and transferring the torque of a motor 57 through gears 53, 54, 55, 56 to the drive shafts 52 and to the common rollers 14. Hence, not only does the construction of the lower part of the head 10 in which the common rollers 14 are installed become complicated but the operation of the common rollers 14 must include the control of the driving of the common rollers 14.
Further, to move the holder 20 fixedly attached with the feed pipe 5 vertically up or down, the guide rods 24 are formed with the feed screw 35 and the gears in the holder 20 in mesh with the feed screw 35 are driven by a motor. Although this construction allows the vertical motion of the holder 20 with high precision, its speed is limited, which has prevented the automatic feeding device for the wire electrode 1 from being improved in the feeding speed. The passage through the pullout rollers 12 of the wire electrode 1 in the automatic feeding operation is checked by detecting the difference in speed between the anneal rollers 13 and the pullout rollers 12. Detecting the speed difference precisely, however, is difficult. Although increasing the speed difference for more accurate detection allows detection of automatic feeding with increased precision, it causes break, slackening or deflection of the wire electrode 1.